Sumner County (standard abbreviation: SU) is a county located in the U.S. state of Kansas. The county's population was 24,132 for the 2010 census. Its county seat is Wellington. Sumner County is part of the Wichita, Kansas Metropolitan Statistical Area. It was named in honor of Charles Sumner, a U.S. Senator from Massachusetts (1811 - 1874), who was a strong advocate of Kansas becoming a free state.